| Abstract: | Investigations of DNA using CD spectroscopy show that the P-form is available in a wide variety of methanol–ethanol mixtures when the water content is low. Increasing the temperature or the ethanol content of a 95% methanol solution causes DNA to undergo a cooperative transition to the P-form. However, this transition cannot be reversed on cooling, or on adding methanol. Thus P-form DNA appears to be stable at high methanol concentrations, but it is usually not observed because the DNA is trapped by a kinetic barrier. P-form DNA will instantaneously assume the native B-form on addition of water, confirming earlier reports that P-form DNA is not strand separated [E. Kay (1976) Biochemistry 15 , 5241]. CD spectra extended to 190 nm show that there is no base–base interaction in the P-form. However, the P-form is extremely stable to heat denaturation in solvents which promote hydrogen bonding between the base pairs. A number of models that can account for the properties of P-form DNA are discussed. |
